Synthesis of dinuclear zinc alkynyls and their use in catalytic silicon-carbon bond formation

Article abstract of DOI:10.1021/om100338s

Zincation of bis(2-pyridylmethyl)amine (dipicolylamine) and phenylacetylene with dimethylzinc yields dimeric [(PhC≡CZn){μ-N(CH₂Py) ₂}]₂ (1). The use of bulky bis[bis(trimethylsilyl)methyl] zinc gives the amine adduct [{(Me₃Si)₂CHZn}(C≡CPh) {HN(CH₂Py)₂}] (2). Treatment of 2 with another equivalent of bis[bis(trimethylsilyl)methyl]zinc yields dinuclear [{(Me₃Si) ₂CHZn}₂(μ-C≡CPh){μ-N(CH₂Py) ₂}] (3), which possesses a central four-membered Zn₂NC ring. Alternatively, 3 is accessible by conversion of the hydride [{(Me ₃Si)₂CHZn}₂(μ-H){μ-N(CH ₂Py)₂}] (4) with phenylacetylene. The addition reaction of 3 with benzaldehyde gives the corresponding alkoxide [{(Me₃Si) ₂CHZn}₂{μ-OCH(C≡CPh)Ph}{μ-N(CH ₂Py)₂}] (5). The complexes 1-3 and 5 were characterized in solution by NMR spectroscopy (C₆D₆) and in the solid state by single-crystal X-ray analysis. The electronic structure of complex 3 was examined by DFT methods. The phenylacetylide 3 and the hydride 4 were found to be the key intermediates in the two-step catalytic process of cross-dehydrocoupling of aryl silanes with phenylacetylene, also supported by NMR spectroscopic studies. Phenylsilane reacts with phenylacetylene to afford mono-, di-, and trialkynylphenylsilane. The reaction of diphenylsilane and phenylacetylene mainly leads to diphenyl(phenylethynyl)silane (7), which was isolated in good yields.

Full text of DOI:10.1021/om100338s

3490 Organometallics 2010, 29, 3490–3499
DOI: 10.1021/om100338s
Synthesis of Dinuclear Zinc Alkynyls and Their Use in Catalytic
Silicon-Carbon Bond Formation
€
Marcel Kahnes, Helmar Gorls, and Matthias Westerhausen*
€
Institut fu€r Anorganische und Analytische Chemie, Friedrich-Schiller-Universitat Jena,
August-Bebel-Strasse 2, 07743 Jena, Germany
Received April 23, 2010
Zincation of bis(2-pyridylmethyl)amine (dipicolylamine) and phenylacetylene with dimethylzinc
yields dimeric [(PhCtCZn){μ-N(CH2Py)2}]2 (1). The use of bulky bis[bis(trimethylsilyl)methyl]zinc
gives the amine adduct [{(Me3Si)2CHZn}(CtCPh){HN(CH2Py)2}] (2). Treatment of 2 with another
equivalent of bis[bis(trimethylsilyl)methyl]zinc yields dinuclear [{(Me3Si)2CHZn}2(μ-CtCPh){μ-
N(CH2Py)2}] (3), which possesses a central four-membered Zn2NC ring. Alternatively, 3 is accessible
by conversion of the hydride [{(Me3Si)2CHZn}2(μ-H){μ-N(CH2Py)2}] (4) with phenylacetylene. The
addition reaction of 3 with benzaldehyde gives the corresponding alkoxide [{(Me3Si)2CHZn}2-
{μ-OCH(CtCPh)Ph}{μ-N(CH2Py)2}] (5). The complexes 1-3 and 5 were characterized in solu-
tion by NMR spectroscopy (C6D6) and in the solid state by single-crystal X-ray analysis. The
electronic structure of complex 3 was examined by DFT methods. The phenylacetylide 3 and the
hydride 4 were found to be the key intermediates in the two-step catalytic process of cross-
dehydrocoupling of aryl silanes with phenylacetylene, also supported by NMR spectroscopic studies.
Phenylsilane reacts with phenylacetylene to afford mono-, di-, and trialkynylphenylsilane. The
reaction of diphenylsilane and phenylacetylene mainly leads to diphenyl(phenylethynyl)silane (7),
which was isolated in good yields.
Introduction
framework, and also in the pseudo-dimeric bimetallic acet-
ylide D the alkynyl ligands act as μ2 or μ3 bridges.5 Terminal
acetylides were found in some alkali metal zincates with
either a trigonal-planar trialkynyl zincate anion in E6 or a
dinuclear dianion with tetrahedrally coordinated zinc atoms
in F.6,7 Stabilization and isolation of terminal acetylides also
succeed in mononuclear complexes with tetramethylethyle-
nediamine as in G.5 Furthermore, with the bulky radical-
anionic 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene
ligand (dpp-BIAN) the dinuclear alkynyl-bridged complex
[(dpp-BIAN)Zn(μ-CCPh)]2 (H) was isolated,8 whereas the
aryl-substituted β-diketiminate ligands [HC(CMeNAr*)2]-
lead to the formation of the monomeric complex I, with a
terminal phenylacetylide.9 However, no current application
of these complexes has been reported so far, although zinc
alkynyls were found to be catalytically active intermediates
and facilitate, besides carbon-carbon bond construction,
the formation of carbon-silicon bonds in the silylation of
1-alkynes. The resulting silylalkynes are important synthons
In the zinc-mediated enantioselective addition of terminal
alkynes to prochiral carbonyl compounds and imines, zinc
alkynyls are considered to be the key element for the asym-
metric carbon-carbon bond formation.1 Being conveniently
prepared in situ from the reaction of terminal alkynes with
dialkylzinc or Zn(OTf)2, these organozinc reagents tolerate
many functional groups, thus making this method an effec-
tive strategy for the preparation of propargylic alcohols and
amines.2 Despite their routine use in organic synthesis, only a
few zinc alkynyl derivatives were isolated and structurally
characterized (see Scheme 1).
Examples of structurally characterized alkynylzinc com-
pounds include the heterocubane structures A3 and B4 with
M4N4 skeletons. In the cage of C, consisting of a Na2Zn2C4
*To whom correspondence should be addressed. E-mail: m.we@
uni-jena.de.
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Published on Web 07/27/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 16, 2010 3491
Scheme 1. Crystallographically Characterized Alkynylzinc Derivatives
for organic synthesis. Their preparation is routinely accom-
plished by the deprotonation of terminal alkynes with strong
bases such as organolithium or Grignard reagents and sub-
sequent trapping of the resulting metal acetylides with a silyl
electrophile. However, the large nucleophilicity of lithium
and magnesium acetylides limits functional group compat-
ibility. Thus, more attractive, complementary methods were
developed employing in situ generated, less nucleophilic zinc
acetylides. Heating a mixture of 1-alkyne and an excess of tri-
alkylchlorosilane in the presence of an excess of zinc powder10
or in situ prepared activated zinc11 in acetonitrile at 120 °C
leads to the formation of silylalkynes in good to excellent
yields. The use of aminosilanes instead of silyl chlorides
allows to carry out the reaction with excess amounts of zinc
halides.12 Still elevated temperatures are required, which in
some cases lead to reductive side reactions forming alkene
byproducts. A lower temperature is sufficient if Zn(OTf)2
and trialkylsilyl chlorides are employed; an at least stoichio-
metric amount of zinc and an additional equivalent amine
base are also required.13 Recent work demonstrated the use
of catalytic amounts of Zn(OTf)2 when the chloride leaving
group at silicon is exchanged by triflate. Thus the formation
of ZnCl2 is prevented, which is not reactive enough to re-
enter the catalytic cycle.14
under salt-free and metal-poor conditions, is in some cases
highly desired, in particular for the preparation of polyalky-
nylsilyl compounds.15,16 Several examples of catalytic dehy-
drogenative silylation of alkynes using a variety of metal
catalysts have been investigated (eq 1). Late transition metal
catalysts including H2PtCl6/I2,17 [Ir(OMe)(COD)]2 with PAr3
or AsAr3,18 [IrH(H2O)(7,8-benzoquinolinato)(PPh3)2]SbF6,
and RhCl(PPh3)319 have been used, but significantly tend to
catalyze also hydrosilylation reactions, thus producing the
corresponding alkenylsilanes as byproduct. The CuCl/NR3
system led to highly selective dehydrogenative cross-coupling
reactions of hydrosilanes with alkynes at elevated tempera-
20
tures.16 Solid bases such as MgO and CaO15 or KNH2/Al2O3
show a similar selective reactivity and enable a convenient
separation from the reaction mixture. Furthermore, complex
metal hydrides such as LiAlH4, NaAlH4, LiBH4, and LiAlH-
(OtBu)3 exhibit a slightly larger but still moderate catalytic
activity.21 Alkoxides, alkyl compounds, and the amides of
alkali metals or barium as more reactive systems allow working
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3492 Organometallics, Vol. 29, No. 16, 2010
Scheme 2. Synthesis of Heteroleptic Zinc Derivatives 1 and 2 with Terminal Phenylethynyl Groups
Kahnes et al.
under milder conditions. In these cases the highest catalytic
activities were observed for Ba(OR)2.22 Recent advances in
the field of alkaline earth metal catalyzed dehydrogenative
coupling reactions between silanes and terminal alkynes were
reported for the azametallacyclopropane complex Ca(η2-
Ph2CNPh)(hmpa)3,23 showing an analogous structure to
the previously prepared Yb(II) complex.24 Selective catalysis
was observed for both complexes with comparable activities.
cat:
R3SiH þ R0C tCH f R3Si- ðC tCR0Þ
ð1Þ
- H2
We herein report the first example of a dehydrogenative
coupling reaction catalyzed by a dinuclear zinc alkynyl
complex and provide a reasonable mechanism for this
two-step process on the basis of X-ray structures for the
proposed intermediates along with reactivity studies and
NMR investigations.
Figure 1. Molecular structure and numbering scheme of 1. The
hydrogen atoms are omitted for clarity reasons. Symmetry-
related atoms (-x, -y, -z) are marked with an “A”. The
ellipsoids of all atoms represent a probability of 40%. Selected
structural data can be found in Table 1.
Results and Discussion
The molecular structure and numbering scheme of phenyl-
ethynylzinc bis(2-pyridylmethyl)amide (1) are shown in
Figure 1. Due to electrostatic attraction of N2, the endocyc-
lic Zn-N bond lengths of 208.6(2) and 209.3(2) pm in the
central Zn2N2 unit are shorter than the exocyclic Lewis acid-
base interactions to the pyridyl groups. Contrary to the
molecular structure of dimeric alkylzinc bis(2-pyridylmethyl)-
amides,27 both pyridyl groups are coordinating to the zinc
atom with nearly equal Zn-Npy bond distances of 220.9(2)
and 221.2(2) pm, leading to pentacoordinate Zn1 with a
distorted trigonal-bipyramidal environment (bond angles:
Synthesis and Structural Characterization. Terminal zinc
phenylacetylides are accessible by direct metalation of phe-
nylacetylene with dialkylzinc in the presence of a chelating
amine.5 The reaction of equimolar amounts of phenylacetyl-
ene and bis(2-pyridylmethyl)amine (dipicolylamine) with
dialkylzinc in toluene solution strongly depended on the
zinc-bound alkyl group R (see Scheme 2). With R = Me
dimeric phenylethynylzinc bis(2-pyridylmethyl)amide (1)
was obtained, which precipitated from the reaction mixture.
The proposed intermediate 1a resulting from complexation
of a preformed methylzinc phenylethynyl with bis(2-pyridyl-
methyl)amine could not be isolated. Even maintenance of a
low reaction temperature did not significantly slow the
metalation reaction of the acidic amine by the zinc-bound
alkyl group. However, metalation with the bulkier bis[bis-
(trimethylsilyl)methyl]zinc gave the amine adduct 2 in good
yields. Even heating of 2 in a heptane solution gave no
further reaction, and therefore compound 1 was not acces-
sible from this bulky dialkylzinc. As discussed elsewhere, the
lower reactivity and basicity of silyl-substituted dimethylzinc
arises from an altered steric and electronic environment of
the R-carbon atom.25,26
N2-Zn1-C13 155.4(1)°, C13-Zn1-N2a 112.7(1)°, N2-
P
Zn1-N2a 91.7(1)°;
359.8°). The Zn1-C13 distance of
200.7(2) pm is similar to those reported for [(TMEDA)-
Zn(tBu)(CtCPh)] (G) despite tetracoordination in the latter
complex.5 The Zn-CtC bond angle (169.4(2)°) deviates from
linearity, which was also observed for the derivatives A, B, G,
and I as well as the zincate E with terminal ethynyl groups.3-6,9
Compared to free phenylacetylene (118.8(2) pm),28 the CtC
bond length in 1 is slightly elongated (121.0(3) pm).
Along with an elongated CtC bond goes a slight bath-
ochrome shift of the CtC stretching vibration at 2090 cm-1
in the IR spectra of 1 (free phenylacetylene ν(CtC) = 2110
cm-1),29 which is also observed for the terminal phenylethy-
nyl in 2 (2095 cm-1). Furthermore, the IR spectrum of the
(22) Ishikawa, J.; Itoh, M. J. Catal. 1999, 185, 454–461.
(23) Buch, F.; Harder, S. Organometallics 2007, 26, 5132–5135.
(24) (a) Takaki, K.; Komeyama, K.; Takehira, K. Tetrahedron 2003,
59, 10381–10395. (b) Takaki, K.; Kurioka, M.; Kamata, T.; Takehira, K.;
Makioka, Y.; Fujiwara, Y. J. Org. Chem. 1998, 63, 9265–9269.
(25) Westerhausen, M.; Rademacher, B.; Poll, W. J. Organomet.
Chem. 1991, 421 (2-3), 175–188.
(27) Jaime, E.; Kneifel, A. N.; Westerhausen, M.; Weston, J.
J. Organomet. Chem. 2008, 693, 1027–1037.
€
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M. M. Chem. Commun. 1997, 1703–1704.
(29) Integrated Spectral Database System of Organic Compounds
(AIST).
€
ꢀ
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metallics 2010, 29, 3098-3108.

Article
Organometallics, Vol. 29, No. 16, 2010 3493
Figure 2. Molecular structure and numbering scheme of 2. The
hydrogen atoms with the exception of the amino group are
omitted for clarity. The ellipsoids of all atoms represent a proba-
bility of 40%. Selected structural data can be found in Table 1.
Figure 3. Molecular structure and numbering scheme of 3. The
hydrogen atoms are omitted for clarity. The ellipsoids of all
atoms represent a probability of 40%. Selected structural data
can be found in Table 1.
Table 1. Selected Bond Lengths (pm) and Angles (deg) of 1-3 and
5
comparable Zn1-Csp (in 1, 200.7(2) pm; in 2, 199.4(2) pm)
and CtC bond lengths (in 1, 121.0(3) pm; in 2; 121.6(3) pm).
Contrary to 1 the Zn1-Csp-Csp moiety (176.4(2)°) shows a
nearly linear arrangement (Table 1).
1
2
3
5
Zn1-N1
220.9(2) 215.3(2) 211.2(2) 218.9(3)
209.3(2) 216.9(3) 209.6(2) 205.7(3)
221.2(2)
200.7(2) 199.4(2) 205.2(2)
Zn1-N2
Zn1-N3/O1
Zn1-CCPh
Zn1-CHn=1,2
Zn2(Zn1A)-N2
Zn2-N3
Zn2-Csp/O1
Zn2-CHn=1,2
CtC
202.2(2)
The acidic proton at N2 facilitates metalation of 2 with a
second equivalent of bis[bis(trimethylsilyl)methyl]zinc as a
synthetic strategy for the preparation of dinuclear zinc
complexes (see Scheme 2). The reaction, which occurred
only at elevated temperatures due to steric shielding of the
reactive sites, gave the phenylethynyl-bridged complex 3 in
good yields. Alternatively, bis[bis(trimethylsilyl)methylzinc]-
phenylethynylbis(2-pyridylmethyl)amide (3) was also acces-
sible from the reaction of stoichiometric amounts of phenyl-
acetylene with the hydride 4, which has been prepared
according to a previously published procedure.26 Infrared
analysis of compound 3 confirmed the presence of a CtC
stretching vibration at 2073 cm-1. Comparison of this absorp-
tion with those of the terminally bound phenylethynyl
groups in 1 and 2 shows that the alkyne stretching vibration
is shifted to a lower frequency, being synonymous with a
lengthening of the CtC bond as derived from X-ray diffrac-
tion studies.
200.7(2) 202.2(3) 200.9(3)
203.1(2) 208.0(3)
211.6(3) 212.3(3)
208.6(2)
231.6(2) 201.1(2)
199.8(2) 200.6(3)
121.0(3) 121.6(3) 122.2(2) 119.6(5)
291.1(1)
88.3(1)
Zn1 Zn2(A)
292.5(1) 296.4(1)
3 3 3
Zn1-N2-Zn2(Zn1A)
Zn1-Csp/O1-Zn2
N2-Zn1-Csp/O1
90.3(1)
83.9(1)
95.9(1)
89.9(1)
91.5(1)
94.6(1)
87.1(1)
86.8(1)
155.4(1) 98.2(1)
N2-Zn2(Zn1A)-Csp/O1 112.7(1)
N1-Zn1-CHn=1,2
119.5(1) 122.4(1) 121.4(1)
122.8(1) 124.4(1)
N3-Zn2-CHn=1,2
Zn1-Csp-Csp
Zn2-Csp-Csp
169.4(2) 176.4(2) 166.3(2)
96.0(2)
amine adduct 2 reveals a sharp absorption at 3296 cm-1
attributable to the N-H stretching vibration.30 In the mo-
lecular structure of 2, depicted in Figure 2, the central zinc
ion adopts a distorted tetrahedral coordination sphere due to
the small dipicolylamine bite angle (N1-Zn1-N2 78.0(1)°).
In the solid state only one pyridyl group coordinates to the
zinc atom. However, in [D6]benzene solution only one set of
resonances is observed for both pyridyl groups, indicating an
exchange between the coordinated and the free pendent side
arm, which is fast on the NMR time scale.27,31 The compar-
ison of the Zn-N2 bond lengths of 1 and 2 reveals an elon-
gated bond in complex 2 (216.9(3) pm) due to the absence of
electrostatic attractions at the protonated N2. Although the
coordination number of zinc is decreased in 2 (nonbonding
Zn1-N3 distance: 397.1(2) pm), both complexes feature quite
The molecular structure of 3 is illustrated in Figure 3.
Selected bond lengths and angles are provided in Table 1.
Compound 3 exhibits an almost planar four-membered
Zn2NC core with a short transannular Zn Zn separation
3 3 3
of 292.5(1) pm. As a result of the unsymmetrically bridging
R-carbon atom of the phenylethynyl moiety, the Zn1-N2
bond length (209.6(2) pm) also differs from the Zn2-N2
value (203.1(2) pm) by 3%. Compared to the dative Zn1-N1
and Zn2-N3 bonds, which show very similar bond distances
(211.2(2) and 211.6(2) pm), they are somewhat shorter due to
electrostatic attractions between the metal centers and N2.
The distances of the zinc atoms to the bridging carbon atom
are very different (Zn1-C13 205.2(2) pm, Zn2-C13 231.6(2)
pm). Both are noticeable longer (6-7 pm) than the corre-
sponding Zn-CR bonds in H.8 The Zn1-C13 bond in 3,
which is not identified as a regular covalent bond by NBO
analysis but rather as a donor-acceptor bond where the
electron pair is delivered exclusively by the carbon atom, has
a pure sp character. However, the Zn1-C13-C14 bond angle
(166.3(2)°) deviates from linearity probably as a consequence
€
(30) (a) Olbert, D.; Kalisch, A.; Herzer, N.; Gorls, H.; Mayer, P.;
Yuc, L.; Reiher, M.; Westerhausen, M. Z. Anorg. Allg. Chem. 2007, 633,
893–902. (b) Westerhausen, M.; Bollwein, T.; Mayer, P.; Piotrowski, H. Z.
Anorg. Allg. Chem. 2002, 628, 1425–1432. (c) Mimoun, H.; de Saint
Laumer, J. Y.; Giannini, L.; Scopelliti, R.; Floriani, C. J. Am. Chem. Soc.
1999, 121, 6158–6166.
(31) Westerhausen, M.; Kneifel, A. N.; Lindner, I.; Grcic, J.; Noth,
H. Z. Naturforsch. 2004, 59b, 161–166.
ꢀ
€

3494 Organometallics, Vol. 29, No. 16, 2010
Kahnes et al.
Figure 4. Representation of the frontier molecular orbitals of 3, obtained from DFT (B3LYP/TZVP) calculations.
Scheme 3. Synthesis and Reactivity of Bis[bis(trimethylsilyl)methylzinc]phenylethynylbis(2-pyridylmethyl)amide (3)
of steric constraints arising from the bulky bis(trimethyl-
silyl)methyl groups.
triple bonds of neighboring alkynyl groups in the solid state
and in solution.33 The Zn-alkyne interactions in 3 lack the
back-bonding component.33 Therefore, the Zn2-C13 bond
is rather weak, leading to a 3þ1 coordination pattern (angle
sum of N2-Zn2-N3, N2-Zn2-C28, and N3-Zn2-C28:
342.7°). Balancing this effect, the Zn2-C28 bond (199.8(2)
pm) experiences a slight shortening compared to the Zn1-
C21 bond (202.2(2) pm). Despite the unsymmetric bonding
situation in the solid state, sharp average resonances are
observed for the aromatic ligands as well as the phenylethy-
The long Zn2-C13 bond as well as the almost perpendi-
cular Zn2-C13-C14 bond angle (96.0(2)°) may be inter-
preted as a side-on coordination of the CtC triple bond to
the zinc atom, which also explains the slightly elongated
C13-C14 bond (122.2(2) pm) compared to the length of the
CtC bond in 1. A DFT single-point calculation (B3LYP/
TZVP) based upon atomic coordinates provided by the
crystallographic analysis of 3 indicates that the HOMO-1
and HOMO-3 lobes with π-symmetry are directed toward
the Zn2 center but are mainly localized at the R-carbon and
the CtC bond (see Figure 4). In monomeric complexes of
[(η5-C5H4SiMe3)2Ti(CtCR)2] π-interactions with Lewis acidic
zinc dihalides have been confirmed crystallographically.32
Recently, Vigalok and co-workers reported the presence of
weak π-interactions between organozinc fragments and the
1
nyl and the CH(SiMe3)2 groups in the H and 13C NMR
spectra of 3 that indicate a fast exchange between σ- and
π-coordination of the CtC triple bond in [D6]benzene
solution.
In order to investigate the nucleophilicity of the bridging
phenylethynyl substituent in 3, the complex was reacted with
an equimolar amount of benzaldehyde in heptane solution.
In a slow reaction showing complete conversion after 18 h a
carbon-carbon bond formation led to the corresponding
propargylic alkoxide 5 (see Scheme 3). During the addition
reaction large colorless prisms of this complex suitable for
X-ray diffraction studies deposited from the reaction mixture.
(32) Lang, H.; Mansilla, N.; Rheinwald, G. Organometallics 2001, 20,
1592–1596.
(33) Bukhaltsev, E.; Goldberg, I.; Cohen, R.; Vigalok, A. Organo-
metallics 2007, 26, 4015–4020.

Article
Organometallics, Vol. 29, No. 16, 2010 3495
Table 2. NMR Chemical Shifts (ppm) Measured in C6D6 at 30 °C
for Compounds 1-5
δ
1
2
3
4
5
1H
Pyr 1
Pyr 2
Pyr 3
Pyr 4
CH2N
ZnCHn=1-3
SiMe3
o-Ph
8.43
6.57
6.73
6.38
3.93
8.40
6.51
6.86
6.45
3.74
1.32
0.40
7.65
7.04
6.95
8.38
6.54
6.91
6.70
4.26
-1.23
0.13
7.66
7.06
6.96
8.19
6.48
6.87
6.66
4.30
-1.37
0.10
8.45
6.56
6.93
6.77
4.28
-1.20
0.02
7.54
7.04
6.99
7.74
7.08
6.91
m-Ph
p-Ph
13C
Pyr 1
149.2
122.0
138.2
122.7
163.1
54.6
149.0
123.1
137.2
122.9
156.1
53.2
147.3
122.6
137.8
122.3
160.1
59.4
147.4
122.9
138.1
122.4
160.7
60.4
148.2
122.7
138.2
123.0
160.5
60.1
Pyr 2
Pyr 3
Pyr 4
Pyr 5
CH2N
SiMe3
ZnCHn=1-3
CCPh
CCPh
i-Ph
o-Ph
m-Ph
p-Ph
Figure 5. Molecular structure and numbering scheme of 5. The
hydrogen atoms are omitted for clarity. The ellipsoids of all
atoms represent a probability of 40%.
4.6
1.5
4.2
4.4
-3.0
4.8
-2.3
110.4
124.3
125.1
131.7
128.0
127.8
-2.2
95.7
85.3
124.6
132.1
128.5
128.4
Due to the centrosymmetric space group, the crystal struc-
ture consists of racemic 5. However, in Figure 5 the mole-
cular structure and numbering scheme of only one of the two
enantiomers is depicted. In the planar central Zn2NO unit of
5 the endocyclic Zn-O bond lengths (Zn1-O1 202.2(2) pm
and Zn2-O1 201.1(2) pm) are very similar to those of the
corresponding methoxide [{(Me3Si)2CHZn}2(μ-OMe){μ-N-
(CH2Py)2}] (6) (200.4(2) and 199.9(2) pm).34 In contrast to
the latter complex, showing a pyramidal geometry of the
oxygen bridge, a planar environment (angle sum: 359.9°) is
observed for O1 in 5.
Attempting to liberate the free propargyl alcohol and
concomitantly regain the dinuclear phenylacetylide 3, com-
plex 5 was treated with excess amounts of phenylacetylene at
room temperature. However, no reaction occurred under
these conditions. Addition of catalytic amounts of tert-butyl-
amine enabled proton transfer. According to the data de-
rived from NMR-scale experiments, the terminal phenyla-
cetylide 2 was formed as a result of the protonation of the
bridging amide N2 instead of O1, leaving an alkylzinc alko-
xide species, which has not yet been fully characterized (see
Scheme 3).
In a previous work we demonstrated that bis(alkylzinc)-
alkoxide-bis(2-pyridylmethyl)amides can conveniently be
converted into the corresponding hydrides by treating them
with aryl silanes.26 The reaction of 5 with phenylsilane
proceeded smoothly, yielding the hydride 4. In a slow but
quantitative reaction the hydride 4 could also be recovered
from the dinuclear phenylacetylide 3 when phenyl- or diphe-
nylsilane was used as a hydride source (see Scheme 2). This
reaction sequence prompted us to apply these dinuclear zinc
complexes as catalysts in the dehydrogenative cross-coupling
of terminal alkynes with hydrosilanes (vide infra).
117.4
107.9
129.1
132.3
128.2
125.9
117.9
107.7
129.1
132.0
128.2
125.8
which are shifted upfield. Similarly, the methylene resonances
of the side arm of 1 and 2 are observed at a higher field
compared to the dinuclear complexes 3, 4,26 and 5. The latter
trend is also mirrored in the 13C NMR data of the CH2N
moiety, e.g., 53.2 ppm for 2 and 59.4 ppm for 3. In the proton
and carbon NMR spectra of 2 the methylene and pyridyl
resonances appear as broad signals due to dynamic processes
as discussed above. The 13C NMR resonances of the terminal
phenylethynyl groups of 1 and 2 are almost identical to each
other and match the NMR data reported for G (R = tBu).5
When changing from terminal to bridging coordination
modes in 3, the signal of the R-carbon atom experiences an
unexpected shift toward higher field, whereas the signal of the
β-carbon is significantly shifted downfield (17 ppm).
Catalytic Reactivity. Dinuclear hydride 4 reacts with stoi-
chiometric amounts of phenylacetylene to give 3 and molec-
ular hydrogen. The formed phenylacetylide 3 can again serve
as a precursor for the hydride 4 via its conversion with
phenyl- or diphenylsilane. Thus this reaction sequence can
be extended to the catalytic process of dehydrogenative
coupling of silanes with terminal alkynes, phenylacetylide 3
and hydride 4 being the active intermediates in the two-step
catalytic cycle (Scheme 4). In addition, bis[bis(trimethylsilyl)-
methylzinc]-alkoxide-di(2-pyridylmethyl)amides (5, 634) can
easily be converted to the required hydride 4 in the presence
of aryl silanes. The dehydrogenative coupling catalyzed by a
dinuclear zinc species is a slow, but selective reaction pro-
ceeding smoothly at room temperature. When diphenylsilane
and 1.3 equiv of phenylacetylene were added to a 3.5 mol %
solution of, for example, 3 in C6D6, pure diphenyl(phenyl-
ethynyl)silane 7 was isolated as major component (81%)
after column chromatography. Also a minor amount of di-
phenyldi(phenylethynyl)silane (8) (7% yield) was obtained.
The use of the primary phenylsilane gave a mixture of
mono-, di-, and trialkynylsilanes. The ratio of products
observed in this reaction depended on the silane/alkyne ratio
and primarily on the reaction time. The monoalkynylsilane
was predominantly formed at the beginning, whereas di- and
NMR Spectroscopic Studies. In the 1H NMR spectra (C6D6)
of 1-3 and 5 the resonances of the phenyl protons essentially
show the same chemical shifts despite different coordination
modes of the phenylethynyl moieties (see Table 2). The
Pyr1-Pyr3 resonances are very much alike, too, and, com-
pared to the hydride 4, shifted toward lower field (e.g., Pyr1 δ =
8.38 ppm for 3 and 8.19 ppm for 4). A difference is observed
for the Pyr4 resonances of the terminal alkynyls 1 and 2,
ꢀ
(34) Kahnes, M.; Richthof, J.; Escudero, D.;Gonzalez, L.;Westerhausen,
M. J. Organomet. Chem. 2010, 695, 280–290.

3496 Organometallics, Vol. 29, No. 16, 2010
Scheme 4. Zinc-Catalyzed Dehydrocoupling of Phenylacetylene with Diphenylsilane
Kahnes et al.
trialkynylsilanes were mainly detected in the 1H NMR spectra
after a prolonged reaction time of 2 to 4 weeks. The formation
of higher alkynyl-substituted silanes proceeded much slower
than the monosubstituted ones most likely due to increased
steric shielding of the silane. For the same reason no reaction
was observed for the tertiary silanes Et3SiH and PhMe2SiH.
Attempting to reduce the steric requirements of the catalyst
and thus accelerating the reaction rate, the zinc-bound alkyl
groups were varied to methyl or trimethylsilylmethyl substit-
uents. In both cases the catalyst decomposed after a short
time, as indicated by the precipitation of metallic zinc accom-
panied by the loss of catalytic activity. Furthermore, the
terminal phenylethynylzinc derivative 1 was tested for cataly-
tic application in the reaction of phenylacetylene with phe-
nylsilane but did not show any activity.
To define further points on the energetic profile of the two-
step reaction sequence, density functional theoretical calcu-
lations at the B3LYP/TZVP level of theory were carried out.
The overall reaction (eq 2) was calculated to proceed in a
slightly exergonic (exothermic) manner with a reaction
Gibbs free energy (ΔG) of -3.2 kcal/mol. The calculated
reaction enthalpy is given in parentheses.
Ph2SiH2 þ PhC tCH f Ph2SiHðC tCPhÞð7Þ þ H2v
ΔG ¼ - 3:2ðΔH ¼ - 3:9Þ kcal=mol
ð2Þ
ð3Þ
ð4Þ
4 þ PhC tCH f 3 þ H2v
ΔG ¼ 0:1ðΔH ¼ - 2:8Þ kcal=mol
3 þ Ph2SiH2 f 4 þ Ph2SiHðC tCPhÞ ð7Þ
ΔG ¼ - 3:3ðΔH ¼ - 1:1Þ kcal=mol
In order to spectroscopically follow the behavior of 3 and
4, the reaction of diphenylsilane with phenylacetylene was
conducted as an NMR-scale experiment using a larger
amount (5 mol %) of catalyst. The reaction progress was
monitored in several intervals over three weeks (see Figure 6).
Adding the silane/alkynyl mixture to a solution of 4 in
[D6]benzene a vigorous evolution of hydrogen indicated
Starting the two-step catalytic process with the hydride 4,
this first half of the cycle (eq 3) is characterized by sligthly
positive Gibbs free energy (ΔG = 0.1 kcal/mol), whereas this
step was found to be exothermic (ΔH = -2.8 kcal/mol).
Such deviations arise from overestimated entropy contribu-
tions to the Gibbs free energy, derived from the assumption
that all substrates are treated as ideal gases. However, the
true ΔG value in solution is regarded to be lower and more
likely to be closer to the ΔH value.35 The second half of the
catalytic cycle (eq 4), restoring the hydride 4, in any case goes
along with a release of energy (ΔG = -3.3 kcal/mol, ΔH =
-1.1 kcal/mol).
1
the immediate conversion to 3. However, in the H NMR
spectra recorded after 4 h both the hydride and the phenyl-
ethynyl derivatives of the catalyst could be detected on the
basis of the pyridyl resonances that are in accordance with
the above-described NMR data (cf. Table 2). Both Pyr1
signals show an almost equal intensity, implying that both
catalytic steps proceed with a very similar rate, concluding a
comparable activation barrier. With a decreasing amount of
phenylsilane the recovery of the hydride 3 was slowed,
shifting the ratio to the acetylide side. The alkynyl form of
the catalyst could clearly be detected over the remaining
period, indicating the good stability under these conditions.
After the diphenylsilane is used up small amounts (<10%)
of diphenyldi(phenylethynyl)silane (8) were formed, as shown
by a new resonance appearing at 8.10 ppm.
(35) (a) Tamura, H.; Yamazaki, H.; Sato, H.; Sakaki, S. J. Am. Chem.
Soc. 2003, 125, 16114–16126. (b) Sakaki, S.; Takayama, T.; Sumimoto, M.;
Sugimoto, M. J. Am. Chem. Soc. 2004, 126, 3332–3348. (c) Sumimoto, M.;
Takahama, N. I. T.; Sakaki, S. J. Am. Chem. Soc. 2004, 126, 10457–10471.
(d) Braga, A. A. C.; Ujaque, G.; Maseras, F. Organometallics 2006, 25,
3647–3658. (e) Deglmann, P.; Ember, E.; Hofmann, P.; Pitter, S.; Walter, O.
Chem.—Eur. J. 2007, 13, 2864–2879.

Article
Organometallics, Vol. 29, No. 16, 2010 3497
Figure 6. NMR studies of the dehydrogenative coupling reaction of phenylacetylene with diphenylsilane catalyzed by 5 mol % of 4
yielding Ph2SiH(CtCPh) (7) and Ph2Si(CtCPh)2 (8).
Summary and Conclusions
distilled under argon; deuterated solvents were dried over
sodium, degassed, and saturated with argon. Phenylacetylene
˚
was dried over activated 4 A molecular sieves and destilled under
Mono- and dinuclear zinc phenylacetylides of the compo-
sition [(PhCtCZn){μ-N(CH2Py)2}]2 (1), [{(Me3Si)2CHZn}-
(CtCPh){HN(CH2Py)2}] (2), and [{(Me3Si)2CHZn}2(μ-
CtCPh){μ-N(CH2Py)2}] (3) were synthesized by direct me-
talation of bis(2-pyridylmethyl)amine and phenylacetylene
with dialkylzinc ZnR2 (R = Me, CH(SiMe3)2). In 3 the
alignment of the bridging alkynyl suggests π-interactions
with one of the zinc centers, also supported by DFT calcula-
tions. The reaction of this complex with benzaldehyde was
explored, leading to the propargyl alkoxide 5, which was
characterized crystallographically. Conversion of 3 and 5
with aryl silanes gave the hydride [{(Me3Si)2CHZn}2(μ-
H){μ-N(CH2Py)2}] (4). This hydride 4 and the phenylacety-
lide 3 were found to be the active species in the two-step
catalytic process of dehydrogenative coupling of terminal
alkynes with hydrosilanes. In this slightly exergonic process
phenyl- and diphenylsilane were selectively converted to the
corresponding alkynylsilanes. NMR investigations con-
firmed the presence of both species during the catalysis and
showed that the rate of the hydride regeneration depends on
the nature of the silane. The dinuclear nature of the two
catalytically active complexes with a close nonbonding me-
tal-metal distance seems to be mandatory in order to enable
this catalytic process. Dinuclear catalysts therefore may
enhance the utility in the field of homogeneous catalysis.
argon prior to use. The compounds [{(Me3Si)2CHZn}2(μ-H){μ-
N(CH)2Py)2}] (4) and [{(Me3Si)2CHZn}2(μ-OMe){μ-N(CH)2-
Py)2}] (6) were prepared by a previously published procedure.26,34
All other compounds were used as supplied by the manufac-
turer.
The 1H and 13C{1H} NMR spectra were recorded on a Bruker
AC 400 MHz spectrometer. Mass spectra were obtained on a
Finnigan MAT SSQ 710 system, and IR measurements were
carried out using a Perkin-Elmer System 2000 FTIR. The IR
spectra were taken as Nujol mulls between KBr windows.
Melting and decomposition points were measured with a Stuart
apparatus type SMP3 and are uncorrected. For the elemental
analysis V2O5 was added to the samples in order to enhance
combustion; nevertheless, carbon values are low most probably
due to carbonate and silicon carbide formation during combus-
tion. Due to the sensitivity of the zinc complexes toward
moisture and air, the elemental analysis proved to be challen-
ging.
Synthesis of [(PhCtCZn){μ-N(CH2Py)2}]2 (1). A mixture of
bis(2-pyridylmethyl)amine (0.60 g, 3.0 mmol) and phenylacetyl-
ene (0.31 g, 3.0 mmol) was dissolved in toluene (10 mL) and
cooled to -78 °C. To the stirred solution was added dropwise a
1.2 M solution of dimethylzinc in toluene (2.5 mL, 3.0 mmol).
The reaction mixture turned claret red while methane was slowly
liberated. On warming to rt, a white, microcrystalline solid
precipitated. The solid (0.84 g, 77%) was collected and washed
with small amounts of toluene. Recrystallization from tetrahy-
drofuran and storage at 5 °C afforded 1 as colorless needles.
Yield: 0.75 g, 69%.
Experimental Section
Physical Data of 1. Decomposition above 165 °C. Anal. Calcd
for C40H34N6Zn2 (729.52): C 65.86, H 4.70, N 11.52. Found: C
65.06, H 5.33, N 11.30. 1H NMR (400.25 MHz, C6D6, 300 K): δ
3.93 (s (br), 4H, CH2N), 6.38 (m (br), 2H, Pyr4), 6.57 (m (br),
General Remarks. All manipulations were carried out under
an argon atmosphere using standard Schlenk techniques. The
solvents were dried according to common procedures and

3498 Organometallics, Vol. 29, No. 16, 2010
Kahnes et al.
2H, Pyr2), 6.73 (m (br), 2H, Pyr3), 6.99 (m, 1H, p-Ph), 7.08 (m,
2H, m-Ph), 7.74 (s (br), 2H, o-Ph), 8.43 (d, 3JH-H = 4.4 Hz, 2H,
Pyr1). 13C{1H} NMR (100.65 MHz, C6D6, 300 K): δ 54.6
(CH2N), 107.9 (CCPh), 117.4 (CCPh), 122.0 (Pyr2), 122.7
(Pyr4), 125.9 (p-Ph), 128.2 (m-Ph), 129.2 (i-Ph), 132.3 (o-Ph),
138.2 (Pyr3), 149.2 (Pyr1), 163.1 (Pyr5). MS (EI, m/z, [%]): 93
([C6H7N]þ) [99], 102 ([C8H6]þ) [100], 107 ([C6H7N2]þ) [49]. IR
(Nujol, KBr, cm-1): ν 3205 m, 3069 m, 3033 m, 3014 s, 2925 vs,
2852 vs, 2820 s, 2728 m, 2690 m, 2090 m, 1981 w, 1963 w, 1944 w,
1891 w, 1871 w, 1801 w, 1747 w, 1638 w, 1593 s, 1571 s, 1450 vs,
1371 s, 1339 s, 1280 s, 1237 m, 1202 m, 1173 w, 1146 s, 1094 m,
1068 m, 1045 s, 1011 s, 995 s, 964 m, 906 m, 895 m, 850 w, 809 m,
772 m, 755 s, 730 s, 695 m, 649 m, 635 m, 626 m, 528 s, 490 m.
Synthesis of [{(Me3Si)2CHZn}(CtCPh){HN(CH2Py)2}] (2).
Bis(2-pyridylmethyl)amine (0.60 g, 3.0 mmol) and phenylacetyl-
ene (0.31 g, 3.0 mmol) were dissolved in 3 mL of toluene and
cooled to -78 °C. To the stirred mixture was added dropwise 6.0
mL of a 0.5 M solution of bis[bis(trimethylsilyl)methyl]zinc (3.0
mmol) in toluene. After being warmed to rt, the solution was
stirred for an additional 14 h. The volume of the solution was
reduced under vacuum to one-third of the original volume.
Subsequent addition of 12 mL of heptane led to the immediate
precipitation of a white, microcrystalline solid. The solid was
collected and recrystallized from a toluene/heptane mixture,
affording 2 as colorless prisms. Yield: 1.21 g, 77%.
4.26 (s, 4H, CH2N), 6.54 (m, 2H, Pyr2), 6.70 (d, 3JH-H = 7.6 Hz,
2H, Pyr4), 6.91 (t,3JH-H = 7.6 Hz, 2H, Pyr3), 6.96 (m, 1H,
p-Ph), 7.06 (t, 3JH-H = 7.2 Hz, 2H, m-Ph), 7.66 (d, 3JH-H = 7.2
3
Hz, 2H, o-Ph), 8.38 (d, JH-H = 4.8 Hz, 2H, Pyr1). 13C{1H}
NMR (100.65 MHz, C6D6, 300 K): δ -2.3 (ZnCH(Si(CH3)3)2),
4.2 (CH(Si(CH3)3)2), 59.4 (CH2N), 110.4 (CCPh), 122.3 (Pyr4),
122.6 (Pyr2), 124.3 (CCPh), 125.1 (i-Ph), 127.8 (p-Ph), 128.0 (m-
Ph), 131.7 (o-Ph), 137.8 (Pyr3), 147.3 (Pyr1), 160.1 (Pyr5). MS
(EI, m/z, [%]): 73 ([Me3Si]þ) [40], 129 ([Me4Si2CH]þ) [89], 262
([C12H12N364Zn]þ) [9], 273 ([Me9Si4C2H2]þ) [100], 367 ([Zn-
[CH(SiMe3)2]2 - Me]þ) [83], 422 ([C19H32N3Si264Zn]þ) [12],
524 ([C27H38N3Si264Zn]þ) [6], 644 ([M(64Zn/64Zn) - CCPh]þ)
[4], 731 ([M(64Zn/64Zn) - Me]þ) [1]. IR (Nujol, KBr, cm-1): ν
3079 m, 3057 m, 2922 vs, 2852 vs, 2738 m, 2671 m, 2495 w, 2452
w, 2356 w, 2335 w, 2312 w, 2257 w, 2162 w, 2073 m, 1995 w, 1980
w, 1951 w, 1912 m, 1883 m, 1842 m, 1755 w, 1715 w, 1651 w, 1606
s, 1572 m, 1455 vs, 1377 s, 1339 m, 1284 m, 1240 s, 1189 m, 1154
m, 1126 m, 1101 m, 1050 s, 1021 s, 978 m, 959 w, 916 m, 851 vs,
830 vs, 755 vs, 667 s, 643 s, 608 s, 541 m, 515 m, 490 s.
Synthesis of [{(Me3Si)2CHZn}2{μ-OCH(CtCPh)Ph}{μ-N-
(CH2Py)2}] (5). To a stirred solution of [{(Me3Si)2CHZn}2(μ-
CtCPh){μ-N(CH2Py)2}] (0.21 g, 0.28 mmol) in heptane (5 mL)
was added benzaldehyde (28 μL, 0.28 mmol). Storage of this
solution at rt led to the precipitation of large colorless prisms,
which were collected. Concentration of the mother liquor under
vacuum and storage at 5 °C afforded a further crop of crystals.
Yield: 0.22 g, 92%.
Physical Data of 2. Melting point: 80 °C. Anal. Calcd for
C27H37N3Si2Zn (525.16): C 61.75, H 7.10, N 8.00. Found: C
59.62, H 6.97, N 7.96. 1H NMR (400.25 MHz, C6D6, 300 K): δ
-1.32 (s, 1H, ZnCH(Si(CH3)3)2), 0.40 (s, 18H, CH(Si(CH3)3)2),
3.74 (s (br), 4H, CH2N), 3.80 (s(br), 1H, NH), 6.45 (m (br), 2H,
Pyr4), 6.51 (m (br), 2H, Pyr2), 6.86 (m (br), 2H, Pyr3), 6.95 (m,
Physical Data of 5. Decomposition above 148 °C. Anal. Calcd
for C41H61N3OSi4Zn2 (855.06): C 57.59, H 7.19, N 4.91. Found:
C 57.12, H 7.36, N 4.98. 1H NMR (400.25 MHz, C6D6, 300 K): δ
-1.20 (s, 2H, ZnCH(Si(CH3)3)2), 0.02 (s, 36H, CH(Si(CH3)3)2),
4.28 (s, 4H, CH2N), 6.13 (s, 1H, OCH), 6.56 (m, 2H, Pyr2), 6.77
1H, p-Ph), 7.04 (t, 3JH-H = 7.2 Hz, 2H, m-Ph), 7.65 (d, 3JH-H
=
(d, 3JH-H = 7.6 Hz, 2H, Pyr4), 6.93 (dt, 3JH-H = 7.6Hz, 4JH-H
=
=
7.2 Hz, 2H, o-Ph), 8.40 (d, 3JH-H = 4.8 Hz, 2H, Pyr1). 13C{1H}
NMR (50.33 MHz, C6D6, 300 K): δ -1.5 (ZnCH(Si(CH3)3)2),
4.6 (CH(Si(CH3)3)2), 53.2 (CH2N), 107.7 (CCPh), 117.9 (CCPh),
122.9 (Pyr4), 123.1 (Pyr2), 125.8 (p-Ph), 128.2 (m-Ph), 129.1
(i-Ph), 132.0 (o-Ph), 137.2 (Pyr3), 149.0 (Pyr1), 156.1 (Pyr5). MS
(EI, m/z, [%]): 93 ([C6H7N]þ) [100], 102 ([C8H6]þ) [93], 129
([Me4Si2CH]þ) [30], 273 ([Me9Si4C2H2]þ) [38], 367 ([Zn[CH-
(SiMe3)2]2 - Me]þ) [14], 422 ([M(64Zn) - CCPh]þ) [0.6]. IR
(Nujol, KBr, cm-1): ν 3296 w, 3092 w, 3065 m, 2925 vs, 2852 vs,
2820 s, 2095 w, 1994 vw, 1962 vw, 1879 vw, 1715 w, 1681 w, 1603
m, 1594 m, 1571 m, 1466 vs, 1443 vs, 1377 s, 1342 w, 1309 w, 1279
m, 1238 s, 1203 m, 1189 m, 1173 m, 1150 s, 1103 m, 1083 s, 1051 s,
1031 s, 1019 s, 1008 m, 994 m, 964 w, 937 w, 909 m, 854 vs, 822 vs,
771 m, 756 s, 741 s, 696 m, 676 m, 665 s, 646 m, 635 m, 607 m, 528
m, 475 s.
1.6 Hz, 2H, Pyr3), 6.99 (m, 1H, p-Ph(CtC)), 7.04 (t, 3JH-H
7.2 Hz, 2H, m-Ph(CtC)), 7.15 (m, 1H, p-Ph(CHO)), 7.29 (t,
3
3JH-H = 7.2 Hz, 2H, m-Ph(CHO)), 7.54 (d, JH-H = 7.2 Hz,
2H, o-Ph(CtC)), 7.80 (d, JH-H = 7.2 Hz, 2H, o-Ph(CHO)),
3
3
8.45 (d, JH-H = 4.8 Hz, 2H, Pyr1). 13C{1H} NMR (100.65
MHz, C6D6, 300 K): δ -2.2 (ZnCH(Si(CH3)3)2), 4.8 (CH(Si-
(CH3)3)2), 60.1 (CH2N), 68.4 (OCH), 85.3 (CCPh), 95.7
(CHOCCPh), 122.7 (Pyr2), 123.0 (Pyr4), 124.6 (i-Ph(CC)),
127.7 (p-Ph(CHO)), 128.4 (p-Ph(CtC)), 128.5 (m-Ph(CC),m-
Ph(CHO)), 128.6 (o-Ph(CHO)), 132.1 (o-Ph(CtC)), 138.2
(Pyr3), 145.8 (i-Ph(CHO)), 148.2 (Pyr1), 160.5 (Pyr5). MS (EI,
m/z, [%]): 73 ([Me3Si]þ) [40], 93 ([C6H7N]þ) [22], 102 ([C8H6]þ)
[41], 129 ([Me4Si2CH]þ) [100], 145 ([Me5Si2CH2]þ) [50], 262
([C12H12N364Zn]þ) [12], 273 ([Me9Si4C2H2]þ) [69], 367
([Zn[CH(SiMe3)2]2 - Me]þ) [16], 564 ([C24H34N3OSi264Zn2]þ)
[1], 588 ([C23H40N3OSi364Zn2]þ) [1]. IR (Nujol, KBr, cm-1): ν
3058 m, 3026 m, 2920 vs, 2849 vs, 2746 m, 2681 m, 2461 w, 2312
w, 2089 m, 1987 w, 1971 w, 1955 w, 1906 m, 1840 m, 1813 m,
1757 w, 1709 w, 1649 w, 1606 s, 1571 m, 1454 vs, 1378 s, 1344 m,
1297 m, 1283 m, 1239 s, 1187 m, 1153 m, 1121 m, 1099 m, 1050 s,
1028 s, 970 m, 921 m, 847 vs, 755 vs, 667 s, 643 s, 607 s, 556 m, 527
m, 483 s.
Synthesis of Ph2SiH(CtCPh (7). To a stirred solution of
[{(Me3Si)2CHZn}2(μ-X){μ-N(CH)2Py)2}] (X = OR, H, CtCPh)
(0.11 mmol, 3.5 mol %) in [D6]benzene (3.0 mL) were added
dropwise diphenylsilane (0.56 g, 3.0 mmol) and phenylacetylene
(0.41 g, 4.0 mmol). The reaction progress was monitored by 1H
NMR spectroscopy, showing the absence of the SiH2 resonance
after 4 weeks. Thereafter all volatiles were removed under vacuum,
giving the crude product as a yellow solid (0.91 g). Column
chromatography with hexane/ethyl acetate (5:1) yielded, upon
concentration, 7 as a pale yellow solid. Yield: 0.69 g, 81%.
Physical Data of Ph2SiH(CtCPh) (7). Melting point: 51 °C.
Anal. Calcd for C20H16Si (284.43): C 84.46, H 5.67. Found: C
83.26, H 5.86. 1H NMR (400.25 MHz, C6D6, 300 K): δ 5.56 (s,
1H, SiH), 6.80-6.90 (m, 3H, m/p-Ph(CtC)), 7.08-7.15 (m, 6H,
m/p-Ph(Si)), 7.31-7.36 (m, 2H, o-Ph(CtC)), 7.71-7.77 (m, 4H,
Synthesis of [{(Me3Si)2CHZn}2(μ-CtCPh){μ-N(CH2Py)2}]
(3). Procedure A. To a stirred solution of [{(Me3Si)2CHZn}-
(CtCPh){HN(CH2Py)2}] (0.73 g, 1.39 mmol) in toluene (3 mL)
was added dropwise 2.8 mL of a 0.5 M solution of bis[bis-
(trimethylsilyl)methyl]zinc (1.4 mmol) in toluene at 0 °C. After
being warmed to rt, the mixture was heated to 60 °C for 2 h.
Thereafter the volume of the solution was reduced to one-third
of the original volume. Then 6 mL of heptane was added. Cool-
ing of this solution to 5 °C gave colorless prisms of 3. Yield:
0.75 g, 72%.
Procedure B. To a stirred solution of [{(Me3Si)2CHZn}2(μ-
H){μ-N(CH2Py)2}] (0.58 g, 0.89 mmol) in heptane (10 mL) was
added tropwise phenylacetylene (100 μL, 0.91 mmol). The
mixture was heated to 60 °C. In the process the evolution of
hydrogen gas was observed, which stopped after 30 min. The
volume of the solution thereafter was reduced to half of the
original volume. Cooling of this solution to 5 °C led to the
precipitation of colorless prisms, which were collected. Yield:
0.54 g, 81%.
Physical Data of 3. Melting point: 92 °C. Anal. Calcd for
C34H55N3Si4Zn2 (748.94): C 54.53, H 7.40, N 5.61. Found: C
53.01, H 7.42, N 5.54. 1H NMR (400.25 MHz, C6D6, 300 K): δ
-1.23 (s, 2H, ZnCH(Si(CH3)3)2), 0.13 (s, 36H, CH(Si(CH3)3)2),

Article
Organometallics, Vol. 29, No. 16, 2010 3499
Table 3. Crystal Data and Refinement Details for the X-ray Structure Determination
1
2
3
5
formula
fw/g mol-1
T/°C
C
40H34N6Zn2
C27H37N3Si2Zn
525.15
-90(2)
monoclinic
P21/n
14.9418(4)
11.6911(3)
17.1955(4)
90
106.712(1)
90
2876.94(13)
4
1.212
9.56
20 120
4908
6574/0.0432
0.0871
0.0352
C34H55N3Si4Zn2
748.91
-90(2)
triclinic
P1
8.5511(2)
11.6155(3)
21.8218(6)
89.250(2)
89.696(2)
70.252(1)
2039.81(9)
2
1.219
13.18
14 179
6431
C41H61N3OSi4Zn2
855.03
-140(2)
monoclinic
P21/c
11.9012(4)
21.4948(6)
18.1196(7)
90
90.497(2)
90
4635.1(3)
4
1.225
11.7
32 342
6024
10 621/0.1093
0.1078
0.0481
0.940
0.469/-0.467
772943
729.47
-140(2)
orthorhombic
Pbca
9.9024(2)
14.0656(4)
24.2369(6)
90
cryst syst
space group
˚
a/A
˚
b/A
˚
c/A
R/deg
β/deg
γ/deg
90
90
3
˚
V/A
Z
3375.79(14)
4
1.435
14.6
19 563
2811
3865/0.0712
0.0830
0.0355
1.024
0.384/-0.514
772940
P
F/g cm-3
μ/cm-1
measd data
measd data (I > 2σ(I))
unique data/Rint
wR2 (all data, on F2)a
R1 (I g 2σ(I)a)
sb
9184/0.0289
0.0886
0.0381
0.970
0.321/-0.414
772942
0.999
-3
˚
residual density/e A
CCDC no.
0.287/-0.338
772941
P
P
P
P
a Definition of the R indices: R1 = (
Fo|-|Fc )/ |Fo|; wR2 = { [w(Fo2 - Fc2)2]/ [w(Fo2)2]}1/2 with w-1 = σ2(Fo2) þ (aP)2. b s = { [w(Fo
-
2
Fc2)2]/(No - Np)}1/2
.
o-Ph(Si)). 13C{1H} NMR (100.65 MHz, C6D6, 300 K): δ 87.7
(CCPh), 110.3 (CCPh), 122.9 (i-Ph(CtC)), 128.3 (p-Ph(Si)),
128.5 (m-Ph(Si/CtC)), 129.2 (p-Ph(CtC)), 130.4 (p-Ph(Si)),
132.5 (o-Ph(CtC)/i-Ph(Si)), 135.6 (o-Ph(Si)). MS (EI, m/z,
[%]): 78 ([C6H6]þ) [17], 105 ([C6H5Si]þ) [39], 129 ([C8H5Si]þ)
[40], 178 ([C14H10]þ) [35], 181 ([C12H9Si]þ) [39], 206 ([M -
C6H6]þ) [72], 284 ([M]þ) [100]. IR (Nujol, KBr, cm-1): ν 3067
m, 3053 m, 3019 m, 2999 m, 2972 vs, 2922 vs, 2854 s, 2156 s, 2141
s, 1971 w, 1957 w, 1899 w, 1888 w, 1832 w, 1813 w, 1763 w, 1681
w, 1594 w, 1587 m, 1572 w, 1487 s, 1466 s, 1440 m, 1429 s, 1376 s,
1328 w, 1312 w, 1269 w, 1216 m, 1188 w, 1157 w, 1117 s, 1065 m,
1024 m, 997 m, 920 m, 859 m, 837 vs, 799 vs m, 756 s, 740 s, 730
vs, 694 vs, 622 m, 615 m, 539 m, 483 s, 471 s.
work of Schafer et al.42 was employed for the first-row atoms as
implemented. For the zinc ions a Stuttgart relativistic pseudo-
potential (known as ECP 10 MDF) has been employed.43 All
species found on the hypersurface were characterized as ener-
getic minima via vibrational analyses. Default convergence
criteria were used, and no symmetry was employed in all the
calculations. The relative stabilities are reported as gas phase
Gibbs free energies containing standard thermochemical (298
K) and vibrational corrections. The natural bond orbital anal-
ysis was performed at the same level of theory with the NBO 5.G
program.44 The representations of Kohn-Sham orbitals were
generated using the Gabedit package.45
€
Acknowledgment. We thank the Deutsche Forschungs-
gemeinschaft (DFG, Bonn/Germany) for generous fina-
ncial support. M.K. wishes to express his gratitude to the
Jena Graduate Academy of the Friedrich Schiller Uni-
versity of Jena, Germany, for a Ph.D. grant.
X-ray Structure Determinations. The intensity data for com-
pounds 1-3 and 5 were collected on a Nonius KappaCCD
diffractometer using graphite-monochromated Mo KR radia-
tion. Data were corrected for Lorentz and polarization effects
but not for absorption effects.36,37 Crystyllographic data as well
as structure solution and refinement details are summerized in
Table 3. The structures were solved by direct methods (SHELXS)38
Supporting Information Available: CIF files giving data
collection and refinement details as well as positional coordi-
nates of all atoms. 1H and 13C NMR spectra of compounds 1-3
and 5. This material is available free of charge via the Internet at
http://pubs.acs.org. In addition, crystallographic data (exclud-
ing structure factors) has also been deposited with the Cam-
bridge Crystallographic Data Centre as supplementary publica-
tion CCDC-772940 for 1, CCDC-772941 for 2, CCDC-772942
for 3, and CCDC-772943 for 5. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/datarequest/cif.
2
and refined by full-matrix least-squares techniques against Fo
(SHELXL-97).39 For compound 1 and for the amine group N2
in 2 the hydrogen atoms were located by difference Fourier
synthesis and refined isotropically. All other hydrogen atoms
were included at calculated positions with fixed thermal para-
meters. All non-hydrogen atoms were refined anisotropically.39
XP (SIEMENS Analytical X-ray Instruments, Inc.) was used
for structure representations.
Computational Methodology. All geometry optimizations and
single-point calculations were performed with the gradient-
corrected hybrid B3LYP40 density functional using the Gaus-
sian03 program package.41 The TZVP basis set based on the
(41) Frisch, M. J.; et al. et al. Gaussian 03; Gaussian, Inc.: Wallingford,
CT, 2004.
€
(36) COLLECT, Data Collection Software; Nonius B.V.: The Netherlands,
1998.
(42) Schafer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100,
5829–5835.
(37) Processing of X-Ray Diffraction Data Collected in Oscillation
Mode; Otwinowski, Z.; Minor, W.; Carter, C. W.; Sweet, R. M. Eds.;
Academic Press: New York, Methods in Enzymology, Vol. 276; Macro-
molecular Crystallography, Part A; 1997; pp 307-326.
(38) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467–473.
(39) Sheldrick, G. M. SHELXL-97 (Release 97-2); University of
(43) Dolg, M.; Wedig, U.; Stoll, H.; Preuss, H. J. Chem. Phys. 1987,
86, 866–872.
(44) Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter,
J. E.; Bohmann, J. A.; Morales, C. M.; Weinhold, F. NBO 5.G; Theoretical
Chemistry Institute, University of Wisconsin: Madison, WI, 2001; http://
www.chem.wisc.edu/∼nbo5.
€
Gottingen: Germany, 1997.
(45) Allouche, A. R. Gabedit is a free Graphical User Interface for
computational chemistry packages. It is available from http://gabedit.
sourceforge.net/.
(40) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (b) Lee, C.;
Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789.